交流电场驱动下悬浮磁性纳米颗粒的取向转变与能量耗散
AC Field-driven orientational crossover and energy dissipation in suspended magnetic nanoparticles
AI总结：
本研究结合朗道-栗弗席兹-吉尔伯特方程与磁性纳米颗粒的布朗旋转动力学，揭示了磁流体热疗中悬浮磁性纳米颗粒的取向随电场振幅转变的规律，明确了不同频率下布朗加热与奈尔加热的主导条件，为优化磁流体热疗性能提供了理论依据。
AI中文摘要：
本研究结合朗道-栗弗席兹-吉尔伯特方程与磁性纳米颗粒（MNPs）的布朗旋转动力学，从理论上探究了颗粒通过易轴重新取向在磁流体热疗（MFH）中的作用。结果表明，随着电场振幅增大，易轴的稳态取向会发生场驱动转变，从主要垂直于外加电场变为主要平行或反平行于外加电场。尽管精确的转变场取决于颗粒尺寸和激发频率，但该转变发生在约0.5H_k处，其中H_k为单轴各向异性场。这些取向区域与潜在的微观动力学及相关的MFH性能直接相关，具体表现为切换和非切换滞后循环，分别主要与奈尔磁化反转和布朗颗粒旋转相关。这些耗散机制的相对重要性还取决于频率：在f=1 MHz时，低电场振幅下布朗加热占主导，而高电场下奈尔加热占主导；相比之下，在f=100 kHz时，两种贡献在大部分研究的电场范围内保持相当。
英文摘要：
By combining the Landau--Lifshitz--Gilbert equation with Brownian rotational dynamics of magnetic nanoparticles (MNPs), we theoretically investigate the role of particle rotation through easy-axis reorientation in magnetic fluid hyperthermia (MFH). Our results reveal a field-driven crossover in the stationary orientation of the easy axes, from predominantly perpendicular to predominantly parallel or antiparallel to the applied field as the field amplitude increases. Although the precise crossover field depends on particle size and excitation frequency, it occurs at approximately $0.5H_k$, where $H_k$ is the uniaxial anisotropy field. These orientational regimes are directly linked to the underlying microscopic dynamics and the associated MFH performance through the occurrence of switching and non-switching hysteresis cycles, predominantly associated with Néel magnetization reversal and Brownian particle rotation, respectively. The relative importance of these dissipation mechanisms also depends on frequency: at $f=1$ MHz, Brownian heating dominates at low field amplitudes, whereas Néel heating dominates at high fields. By contrast, at $f=100$ kHz, both contributions remain comparable over most of the investigated field range.
